i) Field of the invention
The present invention relates to a cathode-ray tube with a low reflectivity film, which is formed with a reflectivity reduction film for reducing the reflectivity of an external light on a face plate.
ii) Description of the Related Arts
Recently, a demand for picture quality of a color TV receiver has been increased, and hence a large improvement of contrast efficiency of a cathode-ray tube has been also demanded.
Before explaining this contrast efficiency, firstly, a construction of a fluorescent part of a cathode-ray tube will now be described.
FIG. 1 shows a cross section of the fluorescent surface part of the cathode-ray tube. In FIG. 1, on an internal surface of a face plate 2, a dark color light absorption film 6, a BGR (blue, green and red) three primary color fluorescent material layer 4 and a metal back film 5 are formed for improving the contrast efficiency by reducing reflectivity of an external light of the fluorescent surface.
Now, in the above-described cathode-ray tube, by assuming an emission luminance F.sub.o of the fluorescent surface; an output luminance F.sub.1 of a light passing through the face plate 2; a light transmittance Tp of the face plate 2; a summarized fluorescent film reflectivity Rp of the dark color light absorption film 6, the BGR three color fluorescent material layer 4 and the metal back film 5; an opening rate Tb of the dark color light absorption film 6; an intensity E.sub.o of a light incident to the fluorescent surface; an intensity E.sub.1 of a surface reflection external light reflected by the external surface of the face plate 2; and an intensity E.sub.2 of a fluorescent surface reflection external light coming out of the face plate 2 by reflecting by the internal surface and the fluorescent film of the face plate 2, a contrast index Ct exhibiting the contrast efficiency (strength) can be expressed by the following formula: ##EQU1## wherein EQU F.sub.1 =F.sub.o .multidot.Tb.multidot.Tp (2) EQU E.sub.1 =0.04E.sub.o ( 3) EQU E.sub.2 =(0.96).sup.2 E.sub.o .multidot.Tp.sup.2{ 0.04+(0.96).sup.2 Rp}(4)
In the above-described formulas, since the material of the face plate 2 is glass, the surface reflection at the interface between the air and the vacuum is estimated at 4%. Since E.sub.1, is constant from formula (3), in order to improve the contrast efficiency or the contrast index Ct, it is apparent from formula (1) that F.sub.1 or the output luminance is enlarged or E.sub.2 or the intensity of the fluorescent surface reflection external light is reduced. In order to reduce E.sub.2, it is understood from formula 4 that it is effective to reduce the light transmittance Tp of the face plate 2. Hence, as a method for improving the contrast efficiency of the cathode-ray tube, it is often practiced to reduce the light transmittance Tp of the face plate 2. In this case, there is a disadvantage, that is, the output luminance F.sub.1 of the cathode-ray tube is simultaneously reduced, which is clear from formula (2).
FIG. 2 illustrates optical characteristics of the face plate 2 and the fluorescent surface. In FIG. 2, the BGR indicates relative emission intensity spectral distribution of the emission from the BGR three color fluorescent material layer 4. Further, in FIG. 2, curves II, III, IV and V denote spectral transmittance distribution of the face plate 2 with the glass having a thickness of 13 mm. II is a clear type with approximately 85% of the spectral transmittance of the visible light area; III is a gray type with approximately 69% of the same; IV is a tint type with approximately 50% of the same; and V is a dark gray type with approximately 38% of the same.
On the other hand, the lower the spectral transmittance of the face plate 2, the larger the absorption of the light emitted from the fluorescent surface of the cathode-ray tube at the face plate 2. Thus, the luminance efficiency decreases. This is clear from the relationship between the relative emission intensity spectral distribution of the BGR fluorescent surface and the spectral transmittance shown in FIG. 2. However, as is apparent from formula (4), since E.sub.2 largely decreases, the external light incident to the fluorescent surface of the cathode-ray tube can be effectively removed, and this is advantageous for the contrast. With the tendency of attaching importance to the image quality of the recent color TV receiver, the face plate 2 of the tint type or dark tint type which highly values the contrast efficiency rather than a conventional clear type or gray type which highly values the luminance efficiency, has been much more used.
Further, with the enlarging of the recent cathode-ray tube and the improvement of the luminance efficiency and the focusing efficiency of the same, the voltage to be applied to the fluorescent surface of the cathode-ray tube, i.e., the acceleration voltage of the electron beam has been raised, and a charge-up phenomenon, i.e., electrification caused by accumulation of electric charges on the face part of the color TV receiver has become a large problem. That is, by this charge-up phenomenon, the fine dust in the air is attached to the face part, and the dirt or the like is liable to become conspicuous. As a result, it becomes a cause for making the luminance efficiency of the cathode-ray tube worse. Further, when a viewer comes up to the charged-up face part, a discharge phenomenon occurs to give an unpleasant feel to the viewer. Hence, in addition to the improvement of the contrast efficiency, the demand for preventing the electrification of the face part has been strong.
In order to carry out the charge prevention of the electrification of the face part of the color TV receiver and the further improvement of the contrast efficiency of the image, as shown in FIG. 3, a cathode-ray tube 1 provided with an antistatic light selective absorption film 3 on the external surface of the face plate 2 has been used. This antistatic light selective absorption film 3 is composed of a silica (SiO.sub.2) film and possesses both an antistatic function and a light selective absorption function. For forming such an antistatic light selective absorption film 3, in general, to a base coating of an alcohol solution of a silicon (Si) alkoxide having a function group such as --OH group or --OR group, tin oxide (SnO.sub.2) or indium oxide (In.sub.2 O.sub.3) fine particles as an electrically conductive filler are dispersed and mixed, and then an organic or inorganic dye or pigment is dispersed and mixed to obtain a coating liquid. This coating liquid is applied to the external surface of the face plate 2 of the cathode-ray tube to form a coating film thereon. After the formation of the coating film, in order to obtain a strong film strength, a baking processing of the film at a temperature of 100.degree. to 200.degree. C. is conducted.
FIG. 4 shows a cross section of the antistatic light selective absorption film 3 formed on the external surface of the face plate 2. In FIG. 4, in the structure of this film 3, the organic or inorganic dye or pigment particles 8 and conductive filler particles 9 are dispersed in the porous silica film 7.
FIG. 5 shows the surface voltage change of the face plate part of the cathode-ray tube. In FIG. 5, L and L.sub.1 indicate variation curves of the surface voltage when the power of the cathode-ray tube having no antistatic function is on (L) or off (L.sub.1), and M and M.sub.1 indicate variation curves of the surface voltage when the power of the cathode-ray tube having an antistatic function is on (M) or off (M.sub.1). In the cathode-ray tube having an antistatic function, a conductive film is formed on the external surface of the face plate 2. Further, since this conductive film is connected to the ground, the surface charges are always released to the ground with the result of largely reducing the charge-up.
Then principle for improving the contrast efficiency by the antistatic light selective absorption film 3 will now be described with reference to FIG. 6.
FIG. 6 is the same as FIG. 1 except that an antistatic light selective absorption film 3 is additionally formed on the external surface of the face plate 2.
Further, the optical refraction index of the glass material of the face plate 2 is determined to be almost the same as that of the antistatic light selective absorption film 3. Thus the light reflection at the interface between these two members can almost be ignored. In this case, a contrast efficiency C't can be expressed in the same manner as described above relating to formula (1) by the following formula: ##EQU2## wherein, EQU F'.sub.1 =F.sub.o .multidot.Tb.multidot.Tp.multidot.Tc (6) EQU E.sub.1 =0.04E.sub.o ( 7) EQU E'.sub.2 =(0.96).sup.2 E.sub.o .multidot.Tp.sup.2 .multidot.Tc.sup.2 {0.04+(0.96).sup.2 Rp} (8)
In the above-described formulas, since E.sub.1 is constant, when Tp is also constant, in order to further improve the contrast efficiency C't, it is effective to decrease the light transmittance Tc of the antistatic light selective absorption film 3 according to formulas (5) and (8). In case of the antistatic light selective absorption film 3, by optimizing the spectral transmittance distribution in the visible light area of the antistatic light selective absorption film 3 and the relative emission intensity spectral distribution of the emission from the BGR three color fluorescent material layer 4, the reduction of the output luminance F'.sub.1 represented by formula (6) can be controlled to be as low as possible so as to improve the contrast efficiency C't.
In FIG. 2, a curve I shows one example of the spectral transmittance distribution of the antistatic light selective absorption film 3 formed on the external surface of the face plate 2 of the cathode-ray tube for the above-described purpose.
In FIG. 2, since, when an absorption peak A of the antistatic light selective absorption film 3 exists in a part near a main spectrum wavelength 535 nm to 625 nm of the relative emission intensity spectral distribution of the GR, it becomes disadvantageous in the luminance efficiency of the fluorescent surface of the cathode-ray tube, the absorption peak A of the absorption range is usually determined in a range of 570 nm to 610 by considering the half value width of the absorption range or the like as well.
Since the light having a wavelength in this range is coincident with a relatively high range of visual sense of eyes of a human being, the absorption or removal of the light component in this range from the external light (usually the white light) components is preferable for the contrast efficiency. That is, by setting the peak A of the absorption range to the wavelength range 570 nm to 710 nm which is relatively high as the visual sense of the humans' eyes, and gives influences as small as possible to the emission from the fluorescent surface (small absorption against the emission of the BGR) with regard to the optical properties of the antistatic light selective absorption film 3 of the cathode-ray tube 1, not only the luminance efficiency of the fluorescent surface is maintained but also the external light is effectively absorbed to improve the contrast efficiency.
In this case, the selection of the organic or inorganic dye or pigment having such optical characteristics is very important. In the case of the curve I shown in FIG. 2, the absorption peak A of the absorption range is set to 580 nm. In such a cathode-ray tube with an antistatic light selective absorption film, since the optical characteristics of the organic or inorganic dye or pigment to be mixed with the base coating are relatively broad, concerning the emission of the fluorescent surface, for example, in the case of the green (G) color emission, a tail part in the long wavelength side of the main spectral wavelength or in the case of the red (R) color emission, a subpeak part in the short wavelength side of the main spectral wavelength is absorbed by this antistatic light selective absorption film. Further, the improvement of the emission color tone can be simultaneously carried out.
FIG. 7 shows an intensity E.sub.o =100 of an external light incident to the external surface of various kinds of face plates 2 of a cathode-ray tube, intensities E.sub.1 of the external light reflected by the external surface of the face plates 2, intensities E.sub.2 of the external light reflected by the internal surface of the face plates 2 and the fluorescent surface and coming out of the face plates 2, and rates of the surface reflected external light per the entire reflected external light [{E.sub.1 /(E.sub.1 +E.sub.2)}.multidot.100].
In this case, regarding the intensities E.sub.1 of the surface reflected external light, in case of face plates k, l, m and n all made of glass materials, the external light is reflected by their external surfaces and in case of face plates o and p formed with an antistatic light selective absorption film 3 having approximately the same optical refraction index as that of the glass materials, the external light is reflected by the external surface of the antistatic light selective absorption film 3. Hence, the intensities E.sub.1 are all approximately 4.0. As to the intensities E.sub.2 of the fluorescent surface reflected external light, they depend on the light transmittance of the face plate 2 and the antistatic light selective absorption film 3 formed on its external surface further, as these light selection rates decrease, the intensities E.sub.2 are suddenly reduced. When the measurement and the evaluation are conducted, an incandescent lamp having relative emission intensity spectral distribution shown in FIG. 8 is used as the external light.
As is apparent from FIG. 7, when the light transmittance of the face plate 2 is relatively high like the face plates k and l, E.sub.2 is very high as compared with E.sub.1. That is the value [{E.sub.1 /(E.sub.1 E.sub.2)}.multidot.100] is small, and the influence of E.sub.1 can be ignored. When the light transmittance of the face plate 2 is lowered like the face plates m and n, E.sub.1 and E.sub.2 become close, and the influence of E.sub.1 can not be ignored. When the light absorption film is formed on the external surface of the face plate 2 whose light transmittance is originally low like the face plates o and p, this tendency becomes further remarkable.
In this instance, phenomenally, in order to improve the contrast efficiency of the cathode-ray tube, the lower the light transmittance of the face plate 2 is reduced and further, the lower the light transmittance is reduced by forming the light absorption film on the external surface of the face plate 2, the more the surface external light reflection by the face plate 2 becomes conspicuous. For instance, the face or the like of the viewer, reflected on the face plate part of the cathode-ray tube is clearly seen by the viewer, and the reflected image obstructs the view very much. Further, when the image is kept on for a long time, it can cause eye fatigue. This conspicuous surface external light reflection problem is marked very much. In particular, when the light transmittance of the face plate 2 is less than 50%, and further, when the light absorption film such as the antistatic light selective absorption film 3 is formed on the external surface of such a face plate 2, the problem becomes more serious.
As described above, in the cathode-ray tube, in order to improve the contrast efficiency, the lower the light transmittance of the face plate 2 is reduced and further the lower the light transmittance is reduced by forming the light absorption film on the external surface of the face plate 2, the more the surface external light reflection of the face plate 2 becomes conspicuous. Due to this reflected image, it becomes hard for the viewer to see the TV image, and the eye fatigue is caused to the viewer.